This invention relates to a rubber composition and a pneumatic tire, and more particularly to a technique for obtaining a rubber composition having a good workability and a high modulus of elasticity as well as a pneumatic tire.
Recently, it is demanded to develop the steering stability and gripping property higher than the conventional one even in tire performances as power and running speed of automobiles become higher.
In rubber industry, bismaleimide is practically examined as a cross-linking agent. In JP-A-63-99251 is reported that the heat resistance and the high maneuverability are simultaneously established by compounding a bismaleimide compound with a styrene-butadiene copolymer rubber (SBR). And also, the effect of lowering heat build-up through hydrazide as reported in JP-A-10-139934.
Among various road surface conditions, a wet road surface on a rainy day is slippery, so that the running of automobiles is risky. And also, there is recently a tendency to take a regulation to a braking distance and hence wet-skid performances become increasingly important.
As a conventional technique for improving wet-skid performances, there are a method of shifting Tg of a polymer to a higher temperature side to increase tan xcex4 (0xc2x0 C.), a method of compounding silica to improve wet performances, and so on.
However, in the former method of increasing tan xcex4, it is difficult to simultaneously establish other performances such as dry-skid performances, low rolling resistance and the like. And also, in the latter method of compounding silica, there are disadvantages that a lock xcexc, at wet state (frictional coefficient when a tire is fully locked and slipping 100%) is not improved, and the like.
On the other hand, the applicant has discovered that the wet-skid performances can be improved by using N,Nxe2x80x2-(1,3-phenylene)bismaleimide and silica together. However, it has been confirmed that when compounding N,Nxe2x80x2-(1,3-phenylene)bismaleimide, tan xcex4 at 60xc2x0 C. is not sufficiently lowered, and the rolling resistance becomes slightly high when being used in a tire tread.
Recently, the inventors have discovered that when bismaleimide is compounded with rubber containing not less than 50% by weight of SBR together with silica, not only a heat resistance is improved but also a dynamic storage modulus (Exe2x80x2) becomes higher and the steering stability is improved when such a rubber composition using bismaleimide is used in a tread.
However, when a greater amount of bismaleimide is compounded with SBR-containing rubber as mentioned above, vulcanization reaction is obstructed by bismaleimide and hence a long time is taken in the vulcanization and also a blow point (easiness of bubble generation in rubber) at the same vulcanizing degree becomes higher, so that it is necessary to take further longer vulcanization time in order to conduct the vulcanization without leaving bubbles in a rubber article, and the productivity largely lowers and becomes impracticable.
As regards this problem, it can naturally be considered to use an accelerator quickening the start-up of the vulcanization or so-called ultra-accelerator such as guanidines, dithiocarbamates and xanthates. However, these compounds can quicken the start-up of vulcanization to shorten a burning time (scorch time) of an uncured rubber, but fail to largely improve the blow point.
Therefore, the inventors have investigated a cause of obstructing the vulcanization reaction through bismaleimide and confirmed that a reaction between a vulcanization accelerator and a bismaleimide occurs earlier than the vulcanization reaction particularly in SBR compounded system and hence the vulcanization is obstructed are used. This is particularly more remarkable when thiazole-based and thiuram-based vulcanization accelerators are used. Further, it has been confirmed that a part of bismaleimide is decomposed and gasified due to the reaction between a bismaleimide and a vulcanization accelerator and hence an amount of gas larger than in a usual case is generated in rubber to degrade the blow point.
It is, therefore, an object of the invention to solve the above problems and to provide a technique capable of further improving a merit of bismaleimide for the increase in elasticity and avoiding a demerit of productivity due to the delay in vulcanization and to provide a rubber composition capable of providing a pneumatic tire with a remarkable non-slip effect, excellent wet-skid performances and a low rolling resistance on a wet road surface having an extremely low frictional coefficient.
In order to achieve the above object, the invention is constituted as follows.
The rubber composition according to the invention is characterized by comprising at least one rubber of natural rubber and synthetic diene rubbers as a rubber ingredient, at least one maleimide compound, and at least one nitrogen-containing compound selected from the group consisting of polyanilines, hydrazides and amine compounds. And also, the rubber composition is characterized by comprising at least one rubber of natural rubber and synthetic diene rubbers and at least one maleimide compound selected from the group consisting of bismaleimide, oligomers and polymers of bismaleimide.
Further, the pneumatic tire according to the invention uses the above rubber composition in its tread.
The invention will be described in detail below.
In the invention can be used natural rubber and synthetic diene rubbers. As the synthetic diene rubber, mention may be made of cis-1,4-polyisoprene, styrene-butadiene copolymer, polybutadiene, low cis-1,4-polybutadiene, high cis-1,4-polybutadiene, ethylene-propylene-diene terpolymer, chloroprene, halogenated butyl rubber, acrylonitrile-butadiene rubber and so on. The natural rubber and synthetic diene rubbers may be used alone or in a blend thereof.
The natural rubber and at least one of cis-1,4-polyisoprene, styrene-butadiene copolymer and polybutadiene as the diene rubber are preferable because the improving effect by the combination with bismaleimide and hydrazide aiming at the invention is made clear.
And also, it is preferable that the remarkable effect is obtained when at least one of butadiene rubber (BR) and styrene-butadiene copolymer rubber (SBR) is included in the rubber ingredient in an amount of not less than 50% by weight. More preferably, the amount is not less than 80% by weight.
Heretofore, a maleimide compound is applied to a rubber composition containing, for example, natural rubber or isoprene rubber in an amount of not less than 50% by weight of the rubber ingredient. In this case, however, the tensile stress of full region becomes too high due to the compounding of maleimide and the object of the invention can not sufficiently be attained. And also, when butyl rubber is included in an amount of not less than 50% by weight, the increase of tensile stress in a desired region becomes small even in the compounding of the maleimide, so that the object of the invention can not be attained sufficiently likewise the above case.
Comparing BR and SBR with each other, SBR is preferable from a viewpoint of the establishment with dry gripping performance, and further the use of an emulsion polymerized SBR is preferable in view of wear resistance.
As the maleimide compounds used in the invention, a bismaleimide compound represented by the formula [7], or an oligomer or polymer of bismaleimide is preferable.
Moreover, when R9 or R20 in the general formula [7] of the bismaleimide compound is an alkylene group, if the carbon number is not less than 4, the molecular weight of the bismaleimide compound becomes large, so that the increasing effect of dynamic storage modulus aimed at the invention is not obtained at the compounding amount thereof.
As the bismaleimide compounds preferably usable in the invention, mention may be made of N,Nxe2x80x2-(1,2-phenylene) bismaleimide, N,Nxe2x80x2-(1,3-phenylene)bismaleimide, N,Nxe2x80x2-(1,4-phenylene) bismaleimide, N,Nxe2x80x2-(4,4xe2x80x2-diphenylmethane)bismaleimide, 2,2-bis[4-(4-maleimidophenoxy)phenyl] propane, bis(3-ethyl,5-methyl-4-maleimidophenyl)methane and so on. One or more of them may be included in the rubber composition.
As the oligomer or polymer of bismaleimide preferably usable in the invention, mention may be made of (1) polybismaleimide oligomer or polymer obtained by simply oligomerizing or polymerizing bismaleimide, (2) polyaminobismaleimide oligomer or polymer obtained from bismaleimide and diamine, (3) bismaleimide triazine oligomer or polymer obtained from bismaleimide and dicyanate and so on.
And also, the maleimides as a starting material for bismaleimide oligomer or polymer may include N,Nxe2x80x2-(1,2-phenylene)bismaleimide, N,Nxe2x80x2-(1,3-phenylene)bismaleimide, N,Nxe2x80x2-(1,4-phenylene)bismaleimide, 1,1xe2x80x2-(methylene-di-4,1-phenylene)bismaleimide and so on. One or more of them can be included.
Furthermore, the diamines as a starting material for polyamino bismaleimide oligomer or polymer may include p-phenylenediamine, m-phenylenediamine, bis(4-aminophenyl) methane, bis(4-aminophenyl) ether, bis(4-aminophenyl) sulphone, bis(4-aminophenyl) sulfide and so on. One or more of them can be included.
Moreover, the triazines as a starting material for the bismaleimide triazine oligomer or polymer or a starting material thereof may include cyanuric acid, monocyanate, dicyanate and so on.
In the invention, the oligomer or polymer of bismaleimide is favorable to have a softening point of not higher than 120xc2x0 C. This is due to the fact that when the softening point exceeds 120xc2x0 C., it tends to degrade the fracture properties of the rubber composition.
In the invention, the compounding amount of the maleimide compound is preferable to be 0.1-30 parts by weight per 100 parts by weight of the rubber ingredient. When it is less than 0.1 part by weight, the effect by the compounding may not sufficiently be obtained, while when it exceeds 30 parts by weight, it tends to degrade the fracture properties. From the similar viewpoint, the amount is preferably 0.5-10 parts by weight, more preferably 0.5-5.0 parts by weight, more particularly 0.5-3.0 parts by weight.
Particularly, in case of using N,Nxe2x80x2-(4,4-diphenylmethane) bismaleimide, the compounding amount is preferable to be 0.5-4 parts by weight per 100 parts by weight of the rubber ingredient. When it is less than 0.5 part by weight, the improving effect of wet performances is not sufficient, while when it exceeds 4 parts by weight, the hardening of rubber tends to become severe and the cost becomes expensive.
The polyanilines preferably used in the invention includes bisanilines, aromatic polyanilines and the like. As the bisanilines, mention may be made of p-phenylenediamine, m-phenylenediamine, bis(4-aminophenyl) methane, bis(4-aminophenyl)ether, bis(3-aminophenyl) sulphone, bis(4-aminophenyl) sulphone, bis(4-aminophenyl) sulfide, 4,4xe2x80x2-(m-phenylene diisopropylidene) dianiline and so on. The aromatic polyanilines may include aromatic polyanilines represented by the general formula [1] and so on. One or more of them may be included in the rubber composition.
Moreover, when n is 0 or 1 in the polyanilines represented by the formula [1], the modulus of elasticity is not sufficiently improved. And also, the carbon number of R6 is not particularly limited. Further, the possible range of 1 in the aromatic polyanilines represented by the formula [2] is an integer of 0 or more, preferably an integer of 0-80.
Further, the compounding amount of the polyanilines is preferable to be 10-200% by weight of the maleimide compound. When the amount is less than 10% by weight, the synergistic effect by the combination with the maleimide compound is not sufficiently obtained, while when it exceeds 200% by weight, the vulcanization is promoted and the rubber tends to be scorched. From the similar viewpoint, the amount is preferably 20-100% by weight.
As a compound represented by the formula [3] among hydrazides used in the invention, mention may be made of 1-hydroxy, Nxe2x80x2-(1-methylethylidene)-2-naphthoic acid hydrazide, 1-hydroxy, Nxe2x80x2-(1-methylpropylidene)-2-naphthoic acid hydrazide, 1-hydroxy, Nxe2x80x2-(1-methylbutylidene)-2-naphthoic acid hydrazide, 1-hydroxy, Nxe2x80x2-(1,3-dimethylbutylidene)-2-naphthoic acid hydrazide, 1-hydroxy, Nxe2x80x2-(2,6-dimethyl-4-heptylidene)-2-naphthoic acid hydrazide, 1-hydroxy, Nxe2x80x2-(2-furylmethylene)-2-naphthoic acid hydrazide, 3-hydroxy, Nxe2x80x2-(1-methylethylidene)-2-naphthoic acid hydrazide, 3-hydroxy, Nxe2x80x2-(1-methylpropylidene)-2-naphthoic acid hydrazide, 3-hydroxy, Nxe2x80x2-(1-methylbutylidene)-2-naphthoic acid hydrazide, 3-hydroxy, Nxe2x80x2-(1,3-dimethylbutylidene)-2-naphthoic acid hydrazide, 2-hydroxy, Nxe2x80x2-(2,6-dimethyl-4-heptylidene)-3-naphthoic acid hydrazide, 2-hydroxy, Nxe2x80x2-(2-furylmethylene)-3-naphthoic acid hydrazide, isophthalic acid di(1-methylethylidene) hydrazide, isophthalic acid di(1-methylpropylidene) hydrazide, isophthalic acid di(1-methylbutylidene) hydrazide, isophthalic acid di(1,3-dimethylbutylidene) hydrazide, isophthalic acid di(2,6-dimethyl-4-heptylidene) hydrazide, isophthalic acid di(2-furylmethylene) hydrazide, isonicotinic acid (1-methylethylidene) hydrazide, isonicotinic acid (1-methylpropylidene) hydrazide, isonicotinic acid (1-methylbutylidene) hydrazide, isonicotinic acid (2,6-dimethyl-4-heptylidene) hydrazide, isonicotinic acid (1,3-dimethylbutylidene) hydrazide, isonicotinic acid (2-furylmethylene) hydrazide, Nxe2x80x2-(1-methylethylidene)-salicylic acid hydrazide, Nxe2x80x2-(1-methylpropylidene)-salicylic acid hydrazide, Nxe2x80x2-(1-methylbutylidene)-salicylic acid hydrazide, Nxe2x80x2-(1,3-dimethylbutylidene)-salicylic acid hydrazide, Nxe2x80x2-(2,6-dimethyl-4-heptylidene)-salicylic acid hydrazide, Nxe2x80x2-(2-furylmethylene)-salicylic acid hydrazide, Nxe2x80x2-(1-methylethylidene)-benzoic acid hydrazide, Nxe2x80x2-(1-methylpropylidene)-benzoic acid hydrazide, Nxe2x80x2-(1,3-dimethylbutylidene)-benzoic acid hydrazide, Nxe2x80x2-(benzylidene)-benzoic acid hydrazide, Nxe2x80x2-(4-dimethylaminophenylmethylene)-benzoic acid hydrazide, Nxe2x80x2-(4-methoxyphenylmethylene)-benzoic acid hydrazide, Nxe2x80x2-(4-hydroxyphenyl methylene)-benzoic acid hydrazide, Nxe2x80x2-(1-phenylethylidene)-benzoic acid hydrazide, Nxe2x80x2-(diphenylmethylene)-benzoic acid hydrazide, Nxe2x80x2-[1-(2,4-dihydroxyphenyl) benzylidene]-benzoic acid hydrazide, Nxe2x80x2-(2-furylmethylene)-benzoic acid hydrazide, Nxe2x80x2-(1-methylethylidene)-1-naphthoic acid hydrazide, Nxe2x80x2-(1-methylpropylidene)-1-naphthoic acid hydrazide, Nxe2x80x2-(1,3-dimethylbutylidene)-1-naphthoic acid hydrazide, Nxe2x80x2-(benzylidene)-1-naphthoic acid hydrazide, Nxe2x80x2-(4-dimethylaminophenylmethylene)-1-naphthoic acid hydrazide, Nxe2x80x2-(4-methoxyphenylmethylene)-1-naphthoic acid hydrazide, Nxe2x80x2-(4-hydroxyphenyl methylene)-1-naphthoic acid hydrazide, Nxe2x80x2-(1-phenylethylidene)-1-naphthoic acid hydrazide, Nxe2x80x2-(diphenylmethylene)-1-naphthoic acid hydrazide, Nxe2x80x2-(1-(2,4-dihydroxyphenyl) benzylidene)-1-naphthoic acid hydrazide, Nxe2x80x2-(2-furylmethylene)-1-naphthoic acid hydrazide, Nxe2x80x2-(1-methylethylidene)-2-naphthoic acid hydrazide, Nxe2x80x2-(1-methylpropylidene)-2-naphthoic acid hydrazide, Nxe2x80x2-(1,3-dimethylbutylidene)-2-naphthoic acid hydrazide, Nxe2x80x2-(benzylidene)-2-naphthoic acid hydrazide, Nxe2x80x2-(4-dimethylaminophenylmethylene)-2-naphthoic acid hydrazide, Nxe2x80x2-(4-methoxyphenylmethylene)-2-naphthoic acid hydrazide, Nxe2x80x2-(4-hydroxyphenylmethylene)-2-naphthoic acid hydrazide, Nxe2x80x2-(1-phenylethylidene)-2-naphthoic acid hydrazide, Nxe2x80x2-(diphenylmethylene)-2-naphthoic acid hydrazide, Nxe2x80x2-[1-(2,4-dihydroxyphenyl) benzylidene]-2-naphthoic acid hydrazide, Nxe2x80x2-(2-furylmethylene)-2-naphthoic acid hydrazide, Nxe2x80x2-(1-methylethylidene)-propionic acid hydrazide, Nxe2x80x2-(1-methylpropylidene) propionic acid hydrazide, Nxe2x80x2-(1,3-dimethylbutylidene)-propionic acid hydrazide, Nxe2x80x2-(benzylidene) propionic acid hydrazide, Nxe2x80x2-(4-dimethylaminophenylmethylene)-propionic acid hydrazide, Nxe2x80x2-(4-methoxyphenylmethylene) propionic acid hydrazide, Nxe2x80x2-(4-hydroxyphenyl methylene)-propionic acid hydrazide, Nxe2x80x2-(1-phenylethylidene)-propionic acid hydrazide, Nxe2x80x2-(diphenylmethylene)-propionic acid hydrazide, Nxe2x80x2-[1-(2,4-dihydroxyphenyl) benzylidene]-propionic acid hydrazide, Nxe2x80x2-(2-furylmethylene)-propionic acid hydrazide, Nxe2x80x2-(1-methylethylidene)-2-methyl-propionic acid hydrazide, Nxe2x80x2-(1-methylpropylidene)-2-methyl-propionic acid hydrazide, Nxe2x80x2-(1,3-dimethylbutylidene)-2-methyl-propionic acid hydrazide, Nxe2x80x2-(benzylidene), 2-methyl-propionic acid hydrazide, Nxe2x80x2-(4-dimethylaminophenylmethylene), 2-methyl-propionic acid hydrazide, Nxe2x80x2-(4-methoxyphenylmethylene), 2-methyl-propionic acid hydrazide, Nxe2x80x2-(4-hydroxyphenylmethylene), 2-methyl-propionic acid hydrazide, Nxe2x80x2-(1-phenylethylidene), 2-methyl-propionic acid hydrazide, Nxe2x80x2-(diphenylmethylene), 2-methyl-propionic acid hydrazide, Nxe2x80x2-[1-(2,4-dihydroxy phenyl) benzylidene], 2-methyl-propionic acid hydrazide, Nxe2x80x2-(2-furylmethylene), 2-methyl-propionic acid hydrazide, Nxe2x80x2-(1-methylethylidene), 2,2-dimethyl-propionic acid hydrazide, Nxe2x80x2-(1-methylpropylidene), 2,2-dimethyl-propionic acid hydrazide, Nxe2x80x2-(1,3-dimethylbutylidene), 2,2-dimethyl propionic acid hydrazide, Nxe2x80x2-(benzylidene), 2,2-dimethyl-propionic acid hydrazide, Nxe2x80x2-(4-dimethylamino phenylmethylene), 2,2-dimethyl-propionic acid hydrazide, Nxe2x80x2-(4-methoxyphenyl methylene), 2,2-dimethyl-propionic acid hydrazide, Nxe2x80x2-(4-hydroxyphenyl methylene), 2,2-dimethyl-propionic acid hydrazide, Nxe2x80x2-(1-phenylethylidene), 2,2-dimethyl-propionic acid hydrazide, Nxe2x80x2-(diphenylmethylene), 2,2-dimethyl-propionic acid hydrazide, Nxe2x80x2-[1-(2,4-dihydroxyphenyl) benzylidene], 2,2-dimethyl propionic acid hydrazide, Nxe2x80x2-(2-furylmethylene), 2,2-dimethyl-propionic acid hydrazide and so on. One or more of them can be included in the rubber composition.
As a preferable hydrazide among them, there are 1-hydroxy, Nxe2x80x2-(1-methylethylidene-2-naphthoic acid hydrazide, 1-hydroxy, Nxe2x80x2-(1-methylpropylidene-2-naphthoic acid hydrazide, 1-hydroxy, Nxe2x80x2-(1-methylbutylidene)-2-naphthoic acid hydrazide, 1-hydroxy, Nxe2x80x2-(1,3-dimethylbutylidene)-2-naphthoic acid hydrazide, 1-hydroxy, Nxe2x80x2-(2,6-dimethyl-4-heptylidene)-2-naphthoic acid hydrazide, 1-hydroxy-Nxe2x80x2-(2-furylmethylene)-2-naphthoic acid hydrazide, 3-hydroxy, Nxe2x80x2-(1-methylethylidene)-2-naphthoic acid hydrazide, 3-hydroxy, Nxe2x80x2-(1-methylpropylidene)-2-naphthoic acid hydrazide, 3-hydroxy, Nxe2x80x2-(1-methylbutylidene)-2-naphthoic acid hydrazide, 3-hydroxy, Nxe2x80x2-(1,3-dimethylbutylidene)-2-naphthoic acid hydrazide, 2-hydroxy, Nxe2x80x2-[2,6-dimethyl-4-heptylidene]-3-naphthoic acid hydrazide, 2-hydroxy, Nxe2x80x2-(2-furylmethylene)-3-naphthoic acid hydrazide, isophthalic acid-di(1-methylethylidene) hydrazide, isophthalic acid-di(1-methylpropylidene) hydrazide, isophthalic acid-di(1-methylbutylidene) hydrazide, isophthalic acid-di(1,3-dimethylbutylidene) hydrazide, isophthalic acid-di(2,6-dimethyl-4-heptylidene) hydrazide, isophthalic acid-di(2-furylmethylene) hydrazide, isonicotinic acid-(1-methylethylidene) hydrazide, isonicotinic acid-(1-methylpropylidene) hydrazide, isonicotinic acid-(1-methylbutylidene) hydrazide, isonicotinic acid-(2,6-dimethyl-4-heptylidene) hydrazide, isonicotinic acid-(1,3-dimethylbutylidene) hydrazide, isonicotinic acid-(2-furylmethylene) hydrazide, Nxe2x80x2-(1-methylethylidene)-salicylic acid hydrazide, Nxe2x80x2-(1-methylpropylidene)-salicylic acid hydrazide, Nxe2x80x2-(1-methylbutylidene)-salicylic acid hydrazide, Nxe2x80x2-(1,3-dimethylbutylidene)-salicylic acid hydrazide, Nxe2x80x2-(2,6-dimethyl-4-heptylidene)-salicylic acid hydrazide and Nxe2x80x2-(2-furylmethylene)-salicylic acid hydrazide.
Moreover, when s in the formula [3] is not less than 4, the molecular weight of hydrazide becomes large and it is undesirably required to largely increase the compounding amount. When Rxe2x80x2 is outside the definition, the given effect is not sufficiently obtained.
As a compound represented by the formula [4] among the hydrazides used in the invention, mention may be made of semicarbazide, carbohydrazide, oxalic acid dihydrazide, malonic acid dihydrazide, succinic acid dihydrazide, glutaric acid dihydrazide, adipic acid dihydrazide, pimelic acid dihydrazide, suberic acid dihydrazide, azelaic acid dihydrazide, sebacic acid dihydrazide, 1,12-dodecane dicarboxylic acid dihydrazide, 1,16-hexadecane dicarboxylic acid dihydrazide, terephthalic acid dihydrazide, isophthalic acid dihydrazide, 2,6-naphthoic acid dihydrazide, 1,4-naphthoic acid dihydrazide, 2,6-pyridine dihydrazide, 4,4xe2x80x2-bisbenzene dihydrazide, salicylic acid hydrazide, p-hydroxy benzoic acid hydrazide, 1-hydroxy-2-naphthoic acid hydrazide, 2-hydroxy-3-naphthoic acid hydrazide, aminobenzoic acid hydrazide and so on. Among them, adipic acid dihydrazide, isophthalic acid dihydrazide, salicylic acid hydrazide and 2-hydroxy-3-naphthoic acid hydrazide are preferable. One or more of them can be included in the rubber composition.
Even in case of the formula [4] likewise the formula [3], when s is not less than 4, the molecular weight of hydrazide becomes large and it is undesirably required to largely increase the compounding amount, and when R1 is outside the definition, the given effect is not sufficiently obtained.
Moreover, the compounding amount of hydrazide is preferable to be 5-200% by weight of the compounding amount of the maleimide compound. When the amount is less than 5% by weight, the synergistic effect by the combination with the maleimide compound is not obtained, while when it exceeds 200% by weight, vulcanization is promoted and the rubber tends to be scorched. From the similar viewpoint, the amount is preferably 10-100% by weight.
The amine compound used in the invention is as follows.
The specified alkyl group adoptable as R1 in the formulae [5 and 6] for the amine and amine addition salt may be straight, branched or cyclic, and includes various octyl groups, various nonyl groups, various decyl groups, various dodecyl groups, various tetradecyl groups, various hexadecyl groups, various octadecyl groups, various behenyl groups, various octenyl groups, various decenyl groups, oleyl group, cyclooctyl group, cyclododecyl group, various cyclooctenyl groups, various cyclododecenyl groups and so on.
The specified aryl group adoptable as R13 in the formulae [5 and 6] may have an adequate substituent such as straight, branched or cyclic lower alkyl group or the like on the aromatic ring, and includes phenyl group, various tolyl groups, various xylyl groups, xcex1- or xcex2-naphthyl group, various methylnaphthyl groups, various dimethylnaphthyl groups and so on.
The specified aralkyl group adoptable as R13 in the formulae [5 and 6] may have an adequate substituent such as straight, branched or cyclic lower alkyl group or the like on the aromatic ring, and includes benzyl group, various methylbenzyl groups, phenethyl group, various methylphenethyl groups, various naphthylmethyl groups, various (methylnaphthyl)methyl groups and so on.
The specified alkyl group adoptable as R14 and R15 in the formulae [5 and 6] may be straight, branched or cyclic, and includes methyl group, ethyl group, n-propyl group, isopropyl group, various butyl groups, various pentyl groups, various hexyl groups, various octyl groups, various decyl groups, various dodecyl groups, propenyl group, allyl group, various pentenyl groups, various hexenyl groups, various octenyl groups, various decenyl groups, cyclopentyl group, cyclohexyl group, cyclooctyl group, various cyclopentenyl groups, various cyclohexenyl groups, various cyclooctenyl groups and so on.
The specified aryl group adoptable as R14 and R15 in the formulae [5 and 6] may have an adequate substituent such as straight, branched or cyclic lower alkyl group or the like on the aromatic ring, and includes phenyl group, various tolyl groups, various xylyl groups, xcex1- or xcex2-naphthyl group, various methylnaphthyl groups and so on.
The specified aralkyl group adoptable as R14 and R15 in the formulae [5 and 6] may have an adequate substituent such as a straight, a branched or a cyclic lower alkyl group or the like on the aromatic ring, and includes benzyl group, various methylbenzyl groups, phenethyl group, various methylphenethyl groups, various naphthylmethyl groups and so on.
On the other hand, the specified alkyl group adoptable as R16 in the formulae [5 and 6] may be straight, branched or cyclic, and includes various hexyl groups, various octyl groups, various nonyl groups, various decyl groups, various dodecyl groups, various tetradecyl groups, various hexadecyl groups, various octadecyl groups, various behenyl groups, various hexenyl groups, various octenyl groups, various decenyl groups, oleyl group, cyclohexyl group, cyclooctyl group, cyclododecyl group, various cyclohexenyl groups, various cyclooctenyl groups, various cyclododecenyl groups and so on.
The specific aryl group and aralkyl group adoptable as R16 in the formulae [5 and 6] may have an adequate substituent such as a straight, a branched or a cyclic lower alkyl group or the like on the aromatic ring, respectively, and may include the same ones as exemplified as the aryl group and aralkyl group in the above description for R16.
As preferable amines constituting the amine and amine addition salt according to the invention, mention may be made of decylamine, laurylamine, myristylamine, palmitylamine, stearylamine, behenylamine, oleylamine, monomethyl decylamine, monomethyl laurylamine, monomethyl myristylamine, monomethyl palmitylamine, monomethyl stearylamine, monomethyl oleylamine, monoethyl decylamine, monoethyl laurylamine, monoethyl myristylamine, monoethyl palmitylamine, monoethyl stearylamine, monoethyl oleylamine, monopropyl decylamine, monopropyl laurylamine, monopropyl myristylamine, monopropyl palmitylamine, monopropyl stearylamine, monopropyl oleylamine, dimethyl decylamine, dimethyl laurylamine, dimethyl myristylamine, dimethyl palmitylamine, dimethyl stearylamine, dimethyl oleylamine, diethyl decylamine, diethyl laurylamine, diethyl myristylamine, diethyl palmitylamine, diethyl stearylamine, diethyl oleylamine, methylethyl decylamine, methylethyl laurylamine, methylethyl myristylamine, methylethyl palmitylamine, methylethyl stearylamine, methylethyl oleylamine, di(hydroxyethyl) decylamine, di(hydroxyethyl) laurylamine, di(hydroxyethyl) myristylamine, di(hydroxyethyl) palmitylamine, di(hydroxyethyl) stearylamine, di(hydroxyethyl) oleylamine and so on. Among them, tertiary alkyl amines in which each of R14 and R15 is a saturated or an unsaturated alkyl group having a carbon number of 1-8, respectively, and the total carbon number of R13, R14 and R15 is 10-24 are particularly favorable.
On the other hand, as the carboxylic acid constituting the amine addition salt according to the invention, saturated or unsaturated straight chain aliphatic acids in which the carbon number of R16 is 10-20 are preferable. As an example of this carboxylic acid, there are mentioned capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, arachic acid, behenic acid, oleic acid and so on.
The amine addition salt according to the invention is preferable to have a molecular weight of 400-800.
Moreover, the amine addition salt may contain an inorganic salt such as a phosphate, a phosphite or the like in an amount of about 0.1-10% by weight of the amine addition salt for the purpose of increasing strength of flake or grain to prevent fusion likely caused at a high temperature in summer.
Further, the compounding amount of the amine compound is preferable to be 50-400% by weight of the maleimide compound. When the amount is less than 50% by weight, the synergistic effect by the combination with the maleimide compound is not obtained, while when it exceeds 400% by weight, vulcanization is promoted and the rubber tends to be scorched. From the similar viewpoint, the amount is preferably 100-200% by weight.
And also, the vulcanization accelerator usable in the invention is not particularly limited, but is preferably a benzothiazole-based vulcanization accelerator such as MBT (2-mercaptobenzothiazole), DM (dibenzothiazyl disulfide), CBS (N-cyclohexyl-2-benzothiazyl sulfeneamide), TBBS (N-t-butyl-2-benzothiazyl sulfeneamide), TBSI (N-t-butyl-2-benzothiazyl sulfeneimide) or the like; a guanidine-based vulcanization accelerator such as DPG (diphenyl guanidine) or the like; a thiuram-based vulcanization accelerator such as tetraoctyl thiuram disulfide, tetrabenzyl thiuram disulfide, tetramethyl thiuram disulfide, tetraisobutyl thiuram disulfide or the like; and a dithiocarbamate such as zinc dialkyldithiophosphate, zinc dibenzyldithiocarbamate or the like. Among them, DM, CBS, TBBS, tetraoctyl thiuram disulfide, and tetrabenzyl thiuram disulfide are preferable because they exhibit a stable vulcanization system.
When a vulcanization accelerator such as guanidine type such as DPG (diphenyl guanidine), or zinc dialkyldithiophosphate is used together with the above preferable vulcanization accelerator, it is favorable that the amount of thiazoles, thiurams and dithiocarbamates in the vulcanization accelerator is not less than 50% by weight because the rigidity at a low strain aiming at the invention is enhanced.
The amount of the vulcanization accelerator used is preferably 0.1-5 parts by weight, more particularly 0.2-3 parts by weight per 100 parts by weight of the rubber ingredient.
The invention is considerably effective for a rubber composition containing a thiazole based vulcanization accelerator and/or a thiuram based vulcanization accelerator as well as a tire.
In the invention, at least one of silica, carbon black, calcium carbonate, titanium oxide and the like may be used as a reinforcing filler. Among them, silica and carbon black are preferable.
The compounding amount of the reinforcing filler is preferably 20-150 parts by weight per 100 parts by weight of the rubber ingredient. When the amount is less than 20 parts by weight, the fracture properties and wear resistance of the vulcanizate are insufficient, while when it exceeds 150 parts by weight, the workability is not favorable. From the similar viewpoint, it is more preferably 25-80 parts by weight.
And also, the invention is effective when silica is compounded in an amount of 5-60 parts by weight per 100 parts by weight of the rubber ingredient. When the compounding amount of silica is less than 5 parts by weight, it tends to lower the dry gripping performance, while when it exceeds 60 parts by weight, it tends to degrade the workability.
As the carbon black used as the reinforcing filler, there are mentioned HAF, ISAF, SAF and so on.
As silica, precipitated silica is preferably used. Particularly, it has a nitrogen adsorption specific surface area (N2SA) of 100 to 300 m2/g, preferably 100 to 250 m2/g.
And also, when carbon black and silica are used together, the compounding ratio may arbitrarily be varied in accordance with the compounding purpose.
In addition to the above rubber ingredient, reinforcing filler, bismaleimide and hydrazide, the rubber composition according to the invention may be compounded with a silane coupling agent, vulcanizing agent, vulcanization accelerator, accelerator activator, antioxidant, antiozonant, age resistor, process oil, zinc white (ZnO), stearic acid and the like, which are usually used in the rubber industry, if necessary.
As the vulcanizing agent usable in the invention is mentioned, for example, sulfur and the like. The amount used is 0.1-10 parts by weight, preferably 0.5-5.0 parts by weight as a sulfur content per 100 parts by weight of the rubber ingredient. When the amount is less than 0.1 part by weight, the fracture properties and wear resistance of the vulcanized rubber tend to be degraded, while when it exceeds 10 parts by weight, the rubber elasticity tends to be lost.
In order to ensure an adequate tensile stress, sulfur is compounded in an amount of not less than 1.0 part by weight, more preferably not less than 1.2 parts by weight but not more than 8 parts by weight. When it exceeds 8 parts by weight, there may be problems such that sulfur blooms onto a rubber surface, and the like.
Furthermore, the compounding amount of the vulcanization accelerator including thiazoles, thiurams and dithiocarbamates is not more than 2 times of the compounding weight of sulfur on the total weight. When the vulcanization accelerator is compounded in excess of this amount, the tensile stress at a large deformation increases and hence the sufficient improvement of wet-skid properties aiming at the invention is not obtained.
It is also an essential feature of the invention that the crosslinking system is a sulfur crosslinking system with a specific range. For example, the object of the invention can not be attained by crosslinking using an organic peroxide or a resin.
As the process oil usable in the invention, mention may be made of paraffinic, naphthenic and aromatic ones. The aromatic oils are preferably used in applications regarding the fracture properties and wear resistance as importance, and the naphthenic or paraffinic oils are preferably used in applications regarding the low heat build-up and low temperature properties as importance. The amount used is preferably not more than 100 parts by weight per 100 parts by weight of the rubber ingredient. When it exceeds 100 parts by weight, it tends to degrade the fracture properties and low heat build-up property of the vulcanized rubber.
A preferable embodiment of the invention is a rubber composition comprising 100 parts by weight of the rubber ingredient, 5-60 parts by weight of silica and 0.5-4 parts by weight of N,Nxe2x80x2-(1,3-phenylene) bismaleimide.
When the compounding amount of N,Nxe2x80x2-(1,3-phenylene) bismaleimide is less than 0.5 part by weight, the wet performances are not sufficiently improved, while when it exceeds 4 parts by weight, the hardening tends to become vigorous and the cost becomes undesirably expensive.
Another preferable embodiment of the invention is a rubber composition compounding 0.5-4 parts by weight of N,Nxe2x80x2-(4,4xe2x80x2-diphenylmethane) bismaleimide and silica with 100 parts by weight of rubber ingredient containing not less than 50% by weight of at least one diene rubber of butadiene rubber and styrene-butadiene copolymer rubber.
When the compounding amount of N,Nxe2x80x2-(4,4xe2x80x2-diphenylmethane) bismaleimide is less than 0.5 part by weight, the wet performances are not improved, while when it exceeds 4 parts by weight, the hardening of the rubber becomes vigorous and the cost becomes undesirably expensive.
Further, the dynamic storage modulus (Exe2x80x2) of the rubber composition is preferable to be 7.0-16.0 MPa. When it is less than 7.0 MPa, the rigidity by feeling is insufficient, while when it exceeds 16.0 MPa, the ride comfort tends to be degraded. Similarly, the tensile stress at 300% elongation (M300) is preferable to be 7.0-15.0 MPa. When it is less than 7.0 MPa, the rigidity by feeling is insufficient, while when it exceeds 15.0 MPa, the ride comfort tends to be degraded.
When the rubber composition according to the invention having the excellent properties as mentioned above is applied to a tread, since distribution of ground contact pressure of a tire can be uniformized, it largely contributes to the improvement of wet-skid performances.
In the pneumatic tire according to the invention, the above rubber composition is applied to the tread rubber. The structure of the tread is not particularly limited, and the rubber composition can be applied to one-layer structure comprising one tread rubber layer, a two-layer structure comprising cap rubber layer/base rubber layer, and further a multilayer structure. In any case, it is effective to arrange the rubber composition according to the invention at a ground contact side.
The rubber composition according to the invention is obtained by kneading the rubber ingredient, the maleimide compound, the nitrogen-containing compound, the reinforcing filler and the like described above by using a kneading machine such as rolls, internal mixer or the like, and vulcanizing after the forming. The rubber composition according to the invention can be suitably used in tire applications as a rubber member such as tire tread, under tread, carcass, sidewall, bead portion or the like as well as applications such as rubber vibration isolator, belt, hose and other industrial articles.
When the rubber composition according to the invention having the excellent properties as mentioned above is applied to a tread rubber of a pneumatic tire, since the distribution of ground contact pressure of the tire can be uniformized in addition to the aforementioned effects, the wet-skid performances are remarkably improved.